This invention relates to a device for controlling the motion of a video-screen cursor with a movably held control ball, and in particular to the bearing arrangement for such a control ball whereby the rotary motion of the control ball is converted into electrical signals by appropriate decoding means.
Such a control ball device, commonly known as a "trackball", is used, for example, with a computer to control the motion of a pointer or cursor on the video screen. In typical applications, the cursor is moved to a predefined field on the video screen, and a specific function is then selected by pressing a key.
A trackball device is described, for example, in U.S. Pat. No. 4,575,086, which shows a trackball held on three rollers, with two of the rollers mounted at right angles to each other and having decoding disks on their shafts. The decoding disks are provided with apertures which cooperate with an LED and a photodetector to provide a series of pulses as the rollers rotate. The control ball can be moved in both coordinate directions, with one of the decoding disks recording the motion of the ball in each of the coordinate directions. Hence, the cursor can be moved both up or down and to right or left on the video screen. The total device (control ball, rollers, decoders and electronics) is enclosed in a housing, the top side of which has a circular opening through which a part of the control ball protrudes. The operator can manipulate the control ball by placing his fingers on the part that protrudes through the housing.
The bearing arrangement for the control ball described in U.S. Pat. No. 4,575,086 is typical of those employed in devices of this kind. For example, U.S. Pat. No. 4,505,165, discloses a similar bearing arrangement, with two rollers mounted perpendicular to one another, each with a decoding disc, and a third roller to support the control ball.
This arrangement has the disadvantage that the control ball tends to depart from a precisely vertical or horizontal track if it is allowed to freewheel. That is, if the control ball is moved in one coordinate direction only, perpendicularly to the axis of one of the rollers and parallel to the axis of the other roller, and the control ball is briefly set in motion and then released, the control ball, and thus also the cursor, will deviate from the direction as it progresses. This means that the cursor does not move precisely from left to right across the entire screen, but drifts up or down as it moves across the screen. The same problem occurs in the vertical direction, causing the cursor to drift left or right.
It is an object of the present invention to provide a trackball device that maintains the same motion vector when the control ball freewheels. That is, when the control ball is set in motion in one coordinate direction and released, it does not depart from that direction, and thus also the motion vector of the cursor does not change.
This object is achieved by shaping at least one of the rotatable rolling bodies so that it is in contact with the control ball at at least two contact points. The result is that, once imparted a rotary momentum in a direction perpendicular to the axis of one of the rolling bodies, the control ball retains its initial rotary momentum, i.e. the cursor continues to move in the direction of its initial vector without showing any deviations. This is achieved predominantly because the two contact points on the control ball provide increased resistance to rotation in the non-desired direction of motion.
It is advantageous if both rolling bodies have two contact points with the control ball, so that the components of the motion vector in both coordinate directions are maintained.
Although it is not necessary to mount the rolling bodies connected to the decoders at right angles to one another, it is advantageous to do so, since one of the coordinates on the screen can then be assigned directly to one of the axes without having to perform a vectorial conversion. Moreover, the cursor will usually be moved either vertically or horizontally.
The additional support for the control ball may be provided by a third roller or by two additional rollers, which may be constructed with one contact point or which may also be designed with two contact points in accordance with the present invention.
In a preferred embodiment of the invention, the cross section of the rotatable rolling body is such that the diameter at two locations, situated along the axis of the rolling body and on opposite sides of the minimum clearance point on the axis with respect to the surface of the control ball, is larger than the diameter at the minimum clearance point. Thus the rolling body supports the control ball at two points situated on either side of the perpendicular from the center of the control ball to the axis of the rolling body. When the control ball is placed in position, the rolling body is automatically centered and held in position. This makes it possible for the rotatable rolling body to be mounted without a stop in the axial direction. This permits a considerably simpler mounting to be used, such as a shaft connected to the rolling body inserted into a groove in a support. In comparison, the conventional bearing arrangements require axial stops, for example, the toe-bearing arrangement disclosed in U.S. Pat. No. 4,575,086.
In the preferred embodiment, the rolling body has the shape of a truncated double cone, in which the end faces of smaller diameter abut. In an alternative embodiment, the rolling body comprises two disk-like bodies or spheres mounted on a shaft. In another alternative embodiment the rolling body has a concave surface contour that matches the curvature of the control ball and contacts the control ball at points in a plane passing through the axis of the rolling body and through the center of the control ball.